booting from an external hard drive

i was doing an erase and install to get things cleaned up and when the installer asked me for my disk 2 i realized that disk 2 had a crack in it. bummer, but i do have osx on my external. is there any way i can tell my mac to boot from an external that is connected with a firewire 400? please help i need my mac back.

oh yah and i am in japan and the mac store said they couldn't help, and i should call apple care and order a new cd but i don't really have an adress since i'm just here for a few weeks for business, otherwise i would just get a replacement, so long story short, i need some help.

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